The break was reported at about 4:15 a.m. on Manuel T. Freitas Parkway, across from Munson Park between Las Pavadas and Las Gallinas avenues, Marin Municipal Water District spokeswoman Libby Pischel said.
The pipe that ruptured was initially thought to be a recycled-water irrigation line, but Pischel later clarified that it was actually a service line.
She said 11 nearby homes and Saint Isabella School, a K-8 school located at 1 Trinity Way, are without water service.
An employee who answered the phone at the school this morning said classes have been canceled for the day.
Crews are at the scene working to repair the line, but Pischel said she does not yet have an estimate for when service will be restored.